This invention relates to gas turbine engines and particularly to means for calculating the temperature at the inlet of the turbine.
As is well known in the gas turbine technology the measurement of turbine inlet temperature has long been sought after. The problems with obtaining the actual measurement, amongst others, are the unreliability of temperature sensors, the irregular temperature pattern, etc, associated with a hostile environment. Typically, this value has been predicted or empirically surmised by scheduling fuel as a function of certain predetermined engine operating parameters. For example, fuel controls like the JFC-25, JFC-60 and others manufactured by the Hamilton Standard Division of United Technologies Corporation, schedule fuel flow in accordance with W.sub.f /P.sub.3 .times. P.sub.3 where W.sub.f is fuel in pounds per hour and P.sub.3 is compressor discharge pressure in pounds per square inch. The W.sub.f /P.sub.3 value is manifested as a function of speed in RPM of the rotating machinery and P.sub.3 is directly sensed and these values are directly multiplied to obtain fuel flow for steady state engine operation. For acceleration the W.sub.f /P.sub.3 is manifested as a function of compressor speed and compressor inlet pressure or temperature and the P.sub.3 sensed value is likewise multiplied to limit the fuel flow.
The measurement of the turbine inlet temperature would be a better parameter for temperature limiting than the method described above as well as any other heretofore known system.
This invention contemplates manifesting a turbine inlet temperature value by sensing certain engine parameters and computing them into a substituted value of the turbine inlet temperature which is a close proximity to if not its actual value. The contemplated system incorporates certain constants which can be calibrated in each installation to "zero in" the calculated temperature to the actual temperature. According to this invention air to fuel ratio is computed by measuring fuel flow (W.sub.f), static compressor discharge pressure (P.sub.S3), total compressor discharge pressure (P.sub.3), and total compressor discharge temperature (T.sub.3) which is then computed to provide engine air flow (W.sub.a). Engine fuel flow (W.sub.f) is then ratioed to provide an air to fuel ratio value. This ratio value together with compressor discharge temperature are "plugged" into the empirical burner can temperature rise equation EQU T.sub.4 = T.sub.3 + K.sub.1 (K.sub.2 T.sub.3 + K.sub.3)f/a + K.sub.4
programmed into a computer, (digital or analogue) for calculating the T.sub.4 value. K.sub.1, K.sub.2, K.sub.3 + K.sub.4 are calibration constants and K.sub.4 may be used, in this instance to adjust for deviations between calculated and actual (T.sub.4) temperature values for a given engine installation.